1. Field of the Invention
The present invention relates to improved processes and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures.
2. Description of the Prior Art
In handling multi-component and single component hydrocarbon liquids such as gasoline, distillates, benzene and the like, air-hydrocarbon vapor mixtures are produced which cannot be vented directly to the atmosphere due to the resulting pollution of the environment and fire and/or explosion hazard. As a result, various processes and apparatus have been developed and used for removing hydrocarbon vapors from air-hydrocarbon vapor mixtures whereby the remaining substantially hydrocarbon-free air can be safely vented to the atmosphere. The removed hydrocarbons are normally liquified and recombined with the hydrocarbon liquid from which they were vaporized thereby making their recovery economically advantageous.
A particularly suitable process and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures are described in U.S. Pat. No. 4,276,058 issued to Dinsmore on Jun. 30, 1981. The process, referred to herein as the "standard process", basically comprises flowing an inlet air-hydrocarbon vapor mixture through a first bed of solid adsorbent whereby hydrocarbons are adsorbed on the bed and a residue gas stream comprised of substantially hydrocarbon free air which is vented to the atmosphere is produced. A second bed of solid adsorbent having hydrocarbons adsorbed therein is regenerated by vacuum pumping whereby a major portion of the hydrocarbons are desorbed from the bed and a hydrocarbon rich air-hydrocarbon mixture is produced. Hydrocarbon-free air is injected into the bed being regenerated when high vacuum conditions are reached to strip additional hydrocarbons from the bed. A substantial portion of the hydrocarbons are recovered from the hydrocarbon rich air-hydrocarbon mixture produced during the regeneration.
The flow pattern of the inlet air-hydrocarbon vapor mixture is periodically changed whereby when the bed through which the inlet air-hydrocarbon vapor mixture is flowing becomes loaded with adsorbed hydrocarbons, the mixture is caused to flow through the bed which has just been evacuated and stripped. Before the inlet vapor mixture is switched into the just regenerated bed which contains a vacuum, the bed is pressure equalized by allowing atmospheric air to enter the bed. Substantially simultaneously with the switching of the inlet air-hydrocarbon vapor mixture flow pattern, the bed which has just become loaded with adsorbed hydrocarbons is changed to regeneration, i.e., to being evacuated and stripped.
Another particularly suitable process and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures are described in U.S. Pat. No. 5,154,735 issued to Dinsmore, et al. on Oct. 13, 1992. The process, referred to herein as the "high efficiency process", is similar to the above described standard process in that it comprises the steps of flowing the inlet air-hydrocarbon vapor mixture through a first bed of solid adsorbent whereby hydrocarbons are adsorbed on the bed and a residue gas stream comprised of substantially hydrocarbon-free air which is vented to the atmosphere is produced. A second bed of solid adsorbent having hydrocarbons adsorbed thereon is regenerated by vacuum pumping with a liquid seal vacuum pump whereby hydrocarbons are desorbed from the bed and a hydrocarbon rich air-hydrocarbon mixture is produced. A substantial portion of the hydrocarbons are recovered from the hydrocarbon rich air-hydrocarbon vapor mixture produced during the regeneration. The beds of adsorbent are periodically changed from adsorption to regeneration and vice versa as described above in connection with the standard process.
In accordance with the high efficiency process, the second bed is further evacuated by vacuum pumping with a positive displacement booster pump connected upstream and in series with the liquid seal vacuum pump while continuing to pump with the liquid seal vacuum pump. A relatively high rate of hydrocarbon-free air is also injected into the bed being regenerated when high vacuum conditions are reached to strip additional hydrocarbons from the bed. The further evacuation and higher rate of stripping air result in the bed being regenerated to a greater degree and a very low hydrocarbon content in the substantially hydrocarbon-free air vented to the atmosphere.
Heretofore, the adsorption, equalization and regeneration cycle times of hydrocarbon vapor recovery processes of the types described above have typically been short, e.g., about 17 minutes for adsorption, two minutes for equalization and 15 minutes for regeneration. In applications of the hydrocarbon vapor recovery processes where the flow of the inlet air-hydrocarbon vapor mixture is intermittent, e.g., processes for recovering vaporized gasoline light ends and the like from a mixture thereof with air expelled from tank trucks, the cycle times utilized have generally been the same as described above. While different techniques have been utilized to conserve power during periods when no inlet air-hydrocarbon vapor mixture is available to be processed, such techniques heretofore have generally involved shutting the vapor recovery apparatus down at the end of a cycle and restarting it when additional inlet air-hydrocarbon mixture must be processed. However, because the same cycle times are utilized, when the apparatus is operated the vacuum pump or pumps and other powered equipment constantly run and power is constantly being consumed. Thus, there is a need for improved processes and apparatus for recovering hydrocarbons from air-hydrocarbon vapor mixtures which can be operated for long periods of time during which the inlet flow of the air-hydrocarbon vapor mixture is intermittent without operating the regeneration equipment and consuming power.
Hydrocarbon vapor recovery apparatus of the type described herein have heretofore utilized timers or vacuum transmitters to facilitate the control of certain aspects of the process. For example, U.S. Pat. No. 5,591,254 issued to Gibson on Jan. 7, 1997 discloses a vapor recovery apparatus with an automatic flow control system which includes a vacuum transmitter for monitoring pressure within the apparatus at selected points and controlling the operation of the valves in response thereto. While a vacuum transmitter can be utilized in conjunction with a valve controller as described in the Gibson patent, the vacuum transmitter readings are affected by site elevation and variations in atmospheric pressure which causes a certain amount of inherent inaccuracy in the operation of the system. Thus, there is a need for an improved vapor recovery process and apparatus which are controlled in certain aspects by accurate measurements of the pressure levels within the adsorbent beds being regenerated and related equipment.
Finally, in the heretofore utilized vapor recovery apparatus, a three-phase separator has been utilized in conjunction with the liquid seal vacuum pump. The three-phase separator has generally been a vessel divided into two sections by an internal baffle which allows vapor and condensed hydrocarbons to flow over the top of the baffle into the absorber side of the vessel to which an absorber is attached. Such a three-phase separator is described in the above mentioned U.S. Pat. No. 5,154,735. In order to eliminate carry-over of the vacuum pump seal liquid into the absorber side of the vessel, a full diameter wire mesh mist extractor has been employed on the vacuum pump seal fluid side of the vessel to coalesce the seal fluid and eliminate carry-over. When the mist extractor must be serviced and cleaned, it is necessary to enter the vessel which is time consuming and requires a relatively long shut-down time. Thus, there is a need for an improved three-phase separator for use in hydrocarbon vapor recovery apparatus of the type described herein.